Floating drilling rigs often need riser floatation to maintain safe working conditions. By buoying up the riser, the deck load on the rig can be reduced, and a low center of gravity can be maintained. For this reason, a number of riser floatation devices or riser tensioners, have been proposed. However, those in commercial use have a number of drawbacks.
Slipping conventional riser tensioners is dangerous business, extremely dangerous when on a moving rig in rough weather.
Further, conventional riser tensioners consume large quantities of wire rope, and for that reason have a high operating cost in addition to the high costs of fluid and repair parts used to keep them operational.
Also, conventional riser floatation is expensive, and is bulky to ship. Periodic removal, inspection and reinstallation of conventional floatation is a labor intensive and expensive operation.
It is an object of this invention to provide a floatation device for a marine riser that reduces top tension in the riser.
It is another object of this invention to provide a floatation device for a marine riser that reduces deck load on a drilling platform above the riser.
It is a further object of this invention to provide a floatation device for a marine riser that results in an increase in the life of wire rope life which is used in the drilling operations.
It is another object of this invention to provide a floatation device for a marine riser that enables drilling rigs to operate at greater depths than before.
It is another object of this invention to provide a floatation device for a marine riser that is inexpensive as compared to floatation devices currently in commercial use, and which is lighter and more compact to store and ship.
It is another object of this invention to provide a floatation device for a marine riser which has lower maintenance requirements than currently used floatation devices, and which is easier to inspect and replace.
It is another object of this invention to provide a riser floatation device for which a rig can be upgraded without shipyard modification.
In one embodiment of the invention, there is provided a band-shaped floatation collar for a marine riser. The floatation collar has a longitudinal axis and is formed from a sidewall body having a longitudinally extending slit extending through the sidewall body. The slit enables the floatation collar to be transversely mounted onto the marine riser. The mounting can easily be accomplished without substantial modification of the drilling rig, such as in the moon pool area.
In another embodiment of the invention, there is provided a floatation collar for a drilling riser. The floatation collar has an upper end and a lower end and a longitudinal axis extending from the upper end to the lower end. A first generally cylindrical sidewall surface defines an inside periphery for the flotation collar and is coaxial with the longitudinal axis. A second generally cylindrical sidewall surface defines an outside periphery for the floatation collar and is positioned radially outwardly from the first generally cylindrical sidewall surface and is additionally coaxial with the longitudinal axis. An arcuate upper end closure surface joins the first generally cylindrical sidewall surface with the second generally cylindrical sidewall at the upper end of the collar. An arcuate lower end closure surface joins the first generally cylindrical sidewall surface with the second generally cylindrical sidewall surface at the lower end of the floatation collar. A first generally rectangular panel surface joins the first generally cylindrical sidewall surface, the second generally cylindrical sidewall surface, the arcuate upper end closure surface, and the arcuate lower end closure surface and is positioned in a plane extending near radially from the longitudinal axis. A second generally rectangular panel surface joins the first generally cylindrical sidewall surface, the second generally cylindrical sidewall surface, the arcuate upper end closure surface, and the arcuate lower end closure surface and is positioned closely alongside the first generally cylindrical panel surface. A slit is formed between the first generally rectangular panel surface and the second generally rectangular panel surface.
The collar is preferably formed by a plurality of wall members defining the various surfaces which enables it to be inflated to provide the necessary buoyancy. Using gas to provide the buoyant force is inexpensive and highly efficient. The device can be smaller than foam filled or metal walled buoys of the same lift, lessening drag by ocean currents. By using fabric wall members, the device can be easily shipped, stored, and deployed. The slit facilitates mounting the device on a riser.
In another embodiment of the invention, there is provided a drilling unit comprising a floating drilling rig, a subsea wellhead, and a riser connecting the subsea drilling rig with the subsea wellhead. A floatation collar encircles the riser so as to reduce deck load on the floating drilling rig. The floatation collar comprises a hollow fabric body filled with gas.
In yet another embodiment of the invention, there is provided a method for adding buoyancy to a riser extending beneath a marine drilling rig and into the water. The method is carried out by lowering a first riser section to beneath the drilling rig main deck. A gripping collar is attached to the first riser section. A plurality of straps are attached to the gripping collar. A second riser section is attached to an upper end of the first riser section. The second riser section is lower to beneath the drilling rig main deck. An inflatable collar is positioned around the second riser section. The inflatable collar is attached to the gripping collar via the plurality of straps. The inflatable collar, is inflated and the second riser section carrying the inflatable collar is submerged to provide the buoyancy.
The amount of buoyance is easily adjusted depending on need by varying the number and/or size of the collars employed. As the working depth of the collars increases, the gas pressure required to provide a given amount of lift will increase. However, the pressure difference across the sidewall of the float will remain at low levels, enabling the device to be constructed inexpensively of fabric.